Aerosol indirect effect sensitivities

Simulations were done with the version 3.7.1 of the Weather Research and Forecasting Model (WRF, Skamarock et al., 2008) . The model was run with a 50 km (LR, d01), a 16.6 km (MR, d02) and a 3.3 km (HR, d03) horizontal resolution. It is forced by the Global Forecast System (GFS) model (National Centers for Environmental Prediction National Weather Service, 2000) as initial and boundary conditions. Temperature, humidity, geopotential and velocity components are nudged towards GFS analysis data with a Newtonian-type method using a relaxation coefficient of 5 × 10 −5 s −1. The microphysical scheme used is the Thompson and Eidhammer (2014) scheme which explicitly calculates the number concentrations of aerosols. The latter are represented in a simplified way according to their capacity to nucleate cloud water ("water friendly", WFA) or ice water ("ice friendly", IFA). Aerosol number concentrations are initialized and forced at domain boundaries by a climatology based on Goddard Chemistry Aerosol Radiation and Transport (GOCART) model (Ginoux et al., 2001) simulations. While no surface emissions are applied to IFA, surface emission fluxes are applied to WFA in order to approximately equilibrate the loss of WFA due to scavenging and nucleation. The radiation scheme is RRTMG (Rapid Radiative Transfer Model for General circulation models, Iacono et al., 2008) and uses the cloud water droplets, ice and snow effective radii of the Thompson and Eidhammer (2014) microphysical scheme to resolve the radiative transfer equations. Another climatology of aerosols from Tegen et al. (1997) is used in this radiative scheme and therefore is not affected by any changes in the microphysical aerosol climatology, which enables us to perform sensitivity experiments of the indirect effects of aerosols with fixed aerosol direct effect. The Kain (2004) scheme is used to parameterize convection. The microphysical effects of aerosols are not taken into account explicitly in this parameterization although they can affect convection indirectly through modifications in the temperature or moisture profiles. The model was run to make two extreme simulations in terms of WFA and IFA microphysical concentrations. Both simulations start on April 1 st , 2013 (after one month of spin-up) and end on September 17, 2013. A very high aerosol emission level (1.75 × 10^7 kg.s^−1 for the whole domain) is applied in the first simulation, referred as MAX or polluted simulation and a very low aerosol emission level (1.75 × 10^−4 kg.s^−1 for the whole domain) is applied for the other simulation, referred as MIN or pristine simulation. Although these emission rates are extreme, maximal and minimal value permitted by the microphysics scheme reduce the range of variation of the number of WFA (NWFA) between ∼ 10 cm^−3 and ∼ 10, 000 cm^−3 and of the number of IFA (NIFA) between 0.005 cm^−3 and 10, 000 cm^−3 . Therefore these latter extreme emission rates ensure that both NIFA and NWFA in the MIN (resp. MAX) simulation remain close to their minimal (resp. maximal) permitted values, which corresponds to a 2 × 10^6 factor for NIFA and a 10^3 factor for NWFA between the MAX and the MIN simulations. Another set of MIN and MAX simulations has been performed at a resolution where convection is resolved (3.3 km) and on a smaller domain in Central Europe. An intermediate set of simulations was used to perform one-way nesting between the LR and the HR simulations, ensuring that the LR simulations force the HR simulations at their boundaries. These intermediate simulations were done at 16.6 km of resolution in an intermediate domain and with the same configuration as the LR simulations. The HR simulations have been performed without activating any convection scheme, since horizontal resolution (3.3 km) is sufficient to resolve convection processes, which is the only difference in model configuration between the LR simulations and the HR simulations.

本研究采用天气研究与预报模式（Weather Research and Forecasting Model，WRF，Skamarock等，2008）3.7.1版本开展模拟。模式运行采用三种水平分辨率：50 km（低分辨率，LR，d01）、16.6 km（中分辨率，MR，d02）和3.3 km（高分辨率，HR，d03）。初始场与边界条件由全球预报系统（Global Forecast System，GFS，美国国家环境预报中心国家气象局，2000）提供。温度、湿度、位势高度及速度分量通过牛顿型方法向GFS分析资料逼近，松弛系数为5×10⁻⁵ s⁻¹。

微物理方案采用Thompson和Eidhammer（2014）方案，该方案显式计算气溶胶数浓度。气溶胶根据其成云（“亲水性”，WFA）或成冰（“亲冰性”，IFA）的核化能力以简化方式表征。气溶胶数浓度的初始化及区域边界强迫采用基于戈达德化学气溶胶辐射与传输模式（Goddard Chemistry Aerosol Radiation and Transport，GOCART，Ginoux等，2001）模拟的气候态数据。IFA未施加地表排放，而WFA施加了地表排放通量，以近似平衡其因清除与核化过程导致的损耗。

辐射方案采用RRTMG（通用环流模式快速辐射传输模型，Rapid Radiative Transfer Model for General circulation models，Iacono等，2008），并利用Thompson和Eidhammer（2014）微物理方案中的云滴、冰及雪的有效半径求解辐射传输方程。该辐射方案采用Tegen等（1997）的另一套气溶胶气候态数据，因此不受微物理过程中气溶胶气候态变化的影响，这使得我们能够在固定气溶胶直接效应的条件下开展气溶胶间接效应的敏感性试验。

对流参数化采用Kain（2004）方案。尽管气溶胶可通过改变温度或湿度廓线间接影响对流，但该参数化方案未显式考虑气溶胶的微物理效应。

针对WFA和IFA的微物理浓度，模式开展了两组极端模拟试验。两组模拟均始于2013年4月1日（经过一个月的spin-up过程），止于2013年9月17日。第一组模拟施加极高的气溶胶排放水平（全区域1.75×10⁷ kg·s⁻¹），称为MAX或污染模拟；另一组施加极低的气溶胶排放水平（全区域1.75×10⁻⁴ kg·s⁻¹），称为MIN或清洁模拟。尽管这些排放率处于极端水平，但微物理方案允许的最大值与最小值将WFA数浓度（NWFA）的变化范围限制在约10 cm⁻³至约10000 cm⁻³之间，IFA数浓度（NIFA）的变化范围限制在0.005 cm⁻³至10000 cm⁻³之间。因此，这些极端排放率确保MIN（对应MAX）模拟中的NIFA与NWFA均接近其允许的最小值（对应最大值）——MAX与MIN模拟之间NIFA的差异达2×10⁶倍，NWFA的差异达10³倍。

另一组MIN和MAX模拟在对流可分辨的分辨率（3.3 km）下开展，区域为中欧的一个较小区域。一组中间模拟用于实现LR与HR模拟之间的单向嵌套，确保LR模拟在边界处强迫HR模拟。这些中间模拟在中间区域以16.6 km分辨率开展，配置与LR模拟相同。HR模拟未激活任何对流方案，因为3.3 km的水平分辨率足以分辨对流过程，这是LR与HR模拟在模式配置上的唯一差异。